Slotted high lift systems are a common feature of current high lift wing design and feature a moving element or elements isolated from the main wing element by a specific aerodynamic slot shape. The function of the moving element is to increase circulation around the main wing element by the addition of extra camber. The geometry of the slot is designed to ensure that the pressure distribution around the trailing element—in particular the leading edge pressure peak—is suppressed to prevent flow separation on that element.
The slot geometry formed between the two elements (FIG. 1) has a powerful influence on the flow quality over the trailing element. It is always important to ensure that the slot shape provides satisfactory aerodynamic performance for all usable deployment angles. Often, due to optimising the slot shape for a particular deflection, the resulting slot shape for other angles is not satisfactory and can result in high pressure gradients on the trailing element's leading edge leading to flow separation.
For good aerodynamic performance a slot should be convergent at the trailing edge of the leading element. (FIG. 2). This means that the tangent at the trailing edge of the leading element and the tangent at a point of minimum distance 1 on the trailing element will converge and intersect downstream of the trailing edge of the leading element. For this condition to be satisfied the minimum distance between the two elements must occur at the trailing edge of the leading element. If the minimum distance 2 occurs at a location forward of the trailing edge of the leading element then there will be no intersection of the tangent lines and a divergent slot shape will result (FIG. 3)
A divergent slot shape can result when a high overlap is used as is usually the case for low deployment angles, due to the curvature of the trailing element upper surface and its proximity to the lower surface of the leading element. The diverging exit shape is undesirable because of the flow separation that can occur at the slot exit.
FIGS. 4-6 illustrate this problem in the case of a trailing edge flap. An aircraft wing 1 a comprises a main wing element 2a and a flap 3a. An actuation system (not shown) moves the flap 3a between a retracted position (FIG. 4) to an extended position (FIG. 5) via an intermediate position (FIG. 6). In the extended position the flap is deployed at an angle δ1 and in the intermediate position the flap is deployed at an angle δ2. In the intermediate position, the air gap 4a is highly divergent. That is, the width of the air gap at its outlet 5a is greater than the minimum width of the air gap at point 6a. This results in a high flow velocity through the gap resulting in an adverse pressure gradient further aft and flow separation 7a at the trailing edge of the flap.
FIGS. 12-14 illustrate this problem in the case of a leading edge slat. An aircraft wing 1b comprises a main wing element 2b and a slat 3b. An actuation system (not shown) moves the slat 3b between a retracted position (FIG. 12) to an extended position (FIG. 13) via an intermediate position (FIG. 14). In the intermediate position, the air gap 4b is divergent. That is, the width of the air gap at its outlet 5b is greater than the minimum width of the air gap at point 6b. This results in a high flow velocity through the gap resulting in an adverse pressure gradient in the divergent region further aft resulting in flow separation 7b on the main wing element 2b. 
One approach to solving this problem in the case of a trailing edge flap is described in US 2006/0202089 A1. The main wing element has a pivotable trailing edge which is adjusted (passively by spring tension or actively with the use of an actuator) to keep the width of the air gap constant or convergent.
A problem with the passive adjustment arrangement is that the width of the air gap cannot be precisely controlled, so the gap will change its shape under different loading. Also the flap upper surface must force the pivotable trailing edge into a flat cruise shape, and this can cause erosion of the flap upper surface. A problem with the active adjustment arrangement is that the actuator adds weight and complexity.
EP 1527992 A2 describes an arrangement for generating vortices, in which a bottom surface of the spoiler at the trailing edge of the wing has several longitudinal, elongated grooves. These grooves contain vortex generators which create vortices over the top surface of a flap behind the spoiler. These vortices prevent flow separation from the flap.
The grooves in EP 1527992 A2 have a relatively small width. This means that the air gap between the spoiler and the flap is predominantly defined by the surface of the spoiler between the grooves. As a result the grooves provide little or no aerodynamic improvement in the shape of the air gap
Also, the spoiler in EP 1527992 A2 is relatively thick compared to the depths of the grooves. As a result, any improvement in the shape of the air gap provided by the grooves is minimal.
Finally, the grooves in EP 1527992 A2 terminate before the trailing edge of the spoiler. As a result the grooves have no effect on the shape of the air gap at its exit.